In the manufacture of ordinary paper and paperboard, "sizing" is effected in order to provide the products with resistance to blotting, water resistance, waterproofness, and the like. Such sizing includes "internal sizing" in which a sizing agent is added to a furnish during the papermaking process before a fiber sheet is formed and "surface sizing" in which a sizing agent is applied on the surface of formed fiber sheet.
In internal sizing, there is employed an acid sizing in which sizing is carried out in a pH range of 4.5-6.5 using rosin sizing agents and crude aluminum sulfate. There is also employed, with increasing popularity, a neutral or alkaline papermaking technique using calcium carbonate as a filler. Here, paper is made in a pH range of 6.5-9 where rosin and alum do not work well, and are sized with ketene dimer compounds, substituted cyclic dicarboxylic acid anhydride compounds, copolymers of a cationic monomer and a hydrophobic monomer, cationized petroleum resins, cationized aliphatic amides and the like. Among these, aqueous dispersions of ketene dimer compounds are most widely used because of their superior sizing effect.
Internal sizing, despite widespread popularity, suffers from the disadvantage of not fully staying with the cellulose fibers during the dewatering phase of the papermaking operation, thus incurring significant losses. In so-called closed-loop papermaking operations, the chemicals lost during the dewatering phase are trapped with the water and recirculated to all phases of the papermaking operation. Often, hydrolyzed diketene by-products deposit on various parts of the paper machine, sometimes redepositing onto the paper itself as contamination, or onto the paper machine causing buildup and ultimately down-time for cleanup.
The surface application of hydrophobes is advantageous in that all of the sizing agent is retained on the paper. However, ketene dimer compounds are inherently reactive with water and are difficult to provide as stable aqueous dispersions. It is very difficult to provide an aqueous dispersion sizing agent which is both stable at high temperatures and mechanically stable. To the inventors' knowledge, ketene dimer, by itself, is not used as a surface size on a commercial scale.
In surface sizing, the sizing agent is, on some occasions, required to provide the paper with surface bonding strength and good printability in addition to blotting resistance.
Korean patent publication 89-1974 discloses a method for manufacturing neutral paper and recommends using a surface size such as styrene-maleic anhydride copolymer, alkyl ketene dimer (AKD), polyvinyl alcohol (PVOH), or any combination thereof. The examples do not show the use of AKD and PVOH together.
JP 45-33189 discloses a PVOH resin which is characterized by the fact that a vinyl ester and a higher alkyl ketene dimer are copolymerized and saponified.
JP 60-246896 discloses a sizing agent composition wherein a cationized PVOH is added as an emulsification stabilizer to a sizing agent that is used in neutral paper manufacture, which sizing agent may be alkenyl succinic anhydrides, AKD's, AKD derivatives, and stearic anhydride.
U.S. Pat. No. 5,013,775 discloses a paper sizing composition comprising a ketene dimer and a hydroplilic vinyl polymer containing an alkyl mercaptan having 6-22 carbon atoms.
GB 1,457,428 discloses sizing agents using organic ketene dimers and teaches that emulsions of high ketene dimer content can be prepared without the use of cationic starch by emulsifying the ketene dimer with polyvinyl alcohol together with a relatively small proportion of a secondary emulsifier in the form of sodium lignosulphonate.